Such a method is known from German Patent No. 199 26 305. There, an internal combustion engine is operated with a lean fuel/air mixture, which means nitrogen oxides are stored temporarily in an accumulator-type catalytic converter. In a regeneration phase, the internal combustion engine is operated with a rich fuel/air mixture, which means the stored nitrogen oxides are catalytically converted.
During the storage of nitrogen oxides, a storage efficiency is calculated, with which the accumulator-type catalytic converter stores nitrogen oxides contained in the exhaust gas of the internal combustion engine. This storage efficiency is dependent, inter alia, on an air-mass flow which, however, only represents a substitute for the space velocity of the exhaust gas in the accumulator-type catalytic converter. In column 3, lines 47 through 49 of German Patent No. 199 26 305, it is assumed that this substitution can be made, since the catalytic converter volume is constant.
However, ascertainment of the storage efficiency according to German Patent No. 199 26 305 has proven to be inaccurate.